The absolutely most important thing developers (and anyone involved in the creation, design, sale, marketing, etc. of software) need to know is based on the fact that
it is designed to be used on multiple devices.

Which leads to my point.
People (your users/customers) are starting to use a wider variety of devices. And not only that, they are using them in different/new ways and this requires that the applications which run on them work/behave in different/new ways too.

Windows 8 tells "desktop" developers they MUST start thinking about developing for multiple devices and uses.

Whether it's a 10" tablet, a 15" laptop or a desktop PC with a 32" screen, these different form factors encourage
use in different ways. But that's just an example of where we'll initially see Win8. There's also the even smaller tablet and phone devices. There's table based interfaces. There's wall sized interfaces. There's also many more we probably can't even begin to imagine yet. Even in Sci-Fi films. But one day, probably
sooner than you think, we'll be building apps to run on such devices.

If you've heard me talk at any point in the last 4 years or so, you will have inevitably/hopefully heard me say that
all developers should start learning about developing for mobile NOW as
it will help prepare you for all the plethora of devices they'll inevitably end up building for in the future.

If you're a developer who currently only targets the laptop/desktop environment usage scenario whether for APPS OR WEBSITES and you care about how your skills will meet the future demands of the marketplace.
I can't recommend strongly enough that you should go and learn about HOW developing for mobile is different. I'm not saying you should go and learn how to build apps for Windows Phone, iPhone or Android, etc. What
you need to know is how apps designed for those platforms are different from ones that run on the "desktop".

If you have experience developing apps for the desktop environment you'll bring with you (whether you intend it or not) a number of assumptions and expectations about what your app should look like, how it should work and how people will use it. The good thing about
mobile development is that it
challenges all these common assumptions and so forces you to think differently.
Once you know how to challenge your thinking about these areas regarding mobile development you'll be
prepared and ready to approach developing for other devices, form factors, etc. in a way which will help you create apps which work well on that platform.
In turn this will lead to apps which are easier (possible?) for your users/customers to use. Which can only be good for you/your company. Afterall, if people can't use your software they wont do so for long. This makes it much harder to get repeat business or referals.
Alternatively if your business is based on income from selling support contracts then you probably want (or need?) your software to be bad. So you should probably check out all the things you shouldn't be doing as it would only make your app better.

Side note. You may try and argue that previously PC apps were used on PCs and laptops. But, seriously, can you show me the apps you have been building which allow for these (slightly) different environments. Thought not.